1. Field of the Invention
The invention relates to a hybrid vehicle capable of running using an engine and an electric motor as power sources, and a method of controlling the same.
2. Description of the Related Art
In recent years hybrid vehicles, which are provided with both an engine that outputs torque generated by burning fuel and an electric motor that outputs torque by receiving a supply of power, have come to be used. These vehicles are capable of running by transmitting the torque from the engine and the electric motor to the wheels, and can be driven in any one of several modes by driving or stopping the engine and the electric motor depending on the running state. For example, the wheels can be driven using only the torque from the engine or only the torque from the electric motor. Alternatively, the wheels can be driven using the torque from both the engine and the electric motor. The electric motor can be driven using power (electricity) stored in a battery. When the state-of-charge (SOC) of the battery drops, the engine drives a generator to charge the battery.
In an in-cylinder injection type engine used in this kind of hybrid vehicle, stratified-charge combustion with a lean air-fuel ratio is achieved by injecting fuel into the combustion chamber during the compression stroke, and homogeneous combustion with a stoichiometric air-fuel ratio is achieved by injecting fuel into the combustion chamber during the intake stroke to form a uniform air-fuel mixture. For example, stratified-charge combustion is achieved in the low load operating region of the internal combustion engine by opening an intake valve during the intake stroke of the piston such that air is drawn from an intake port into the combustion chamber where it is compressed during the compression stroke. Fuel is then injected into this high pressure air and the two mix. The resultant air-fuel ratio mixture is led to a spark plug where it is ignited. Meanwhile, homogeneous combustion is achieved in the mid and high load operating regions of the internal combustion engine by opening the intake valve during the intake stroke of the piston such that air is drawn from the intake port into the combustion chamber. As the air is drawn in, fuel is also injected such that an air-fuel mixture is formed which disperses throughout the entire combustion chamber. This dispersed air-fuel mixture is then ignited by a spark plug and burns uniformly throughout the entire combustion chamber.
Japanese Patent Application Publication No. 2005-069029 (JP-A-2005-069029), for example, describes one such hybrid vehicle. Also, Japanese Patent Application Publication No. 2000-130213 (JP-A-2000-130213), for example, describes an in-cylinder injection type engine.
With the engine of the related art described above, when the operating mode changes according to the operating state between an operating mode that realizes stratified-charge combustion with a lean air-fuel ratio and an operating mode that realizes homogeneous combustion with the stoichiometric air-fuel ratio, an operating parameter such as the intake air amount or the ignition timing or the like is changed according to the change in the air-fuel ratio in order to compensate so that the actual output torque does not vary from the torque required by the driver. However, the engine torque adjustment when the air-fuel ratio is changed temporarily becomes off, at which time a shock is generated that may adversely affect drivability.
JP-A-2000-130213 also describes a control apparatus for a lean burn engine. With this control apparatus, even if an opening amount command value output to a throttle actuator is increased or decreased in a stepped manner when a condition for executing lean burn is satisfied or not satisfied, there is a time lag with the throttle actuator, as well as a response delay of the air (due to inertia and distance between the throttle and the combustion space). Therefore, with the control apparatus of the lean burn engine described in JP-A-2000-130213, the shifts in the target air-fuel ratio, ignition timing, and the injection timing are synchronized with the time lag and the response delay of the air. However, the maladjustment of the engine torque when the air-fuel ratio is changed is often due not only to response delays and the like of devices such as an electronic throttle device, but also to secular variation of these devices and deviations in the intake air temperature, as well as differences in the combustion rate due to the temperature difference of the residual gasses. Therefore, this maladjustment of the engine torque is unable to be properly compensated for and as a result, a shock is generated which adversely affects drivability.